On occasion it becomes necessary to replace or augment a natural spinal disc nucleus pulposus with a prosthesis. For example, a spinal disc may become damaged due to trauma or disease resulting in a disc herniation. Such a prosthesis should preferably mimic the shape and function of the natural nucleus pulposus. Various types of prostheses have been designed, including hydrogels in the form of beads and solid implants. For example, U.S. Pat. No. 5,192,326 to Bao et al. discloses a prosthetic nucleus pulposus made of either a solid hydrogel core or a plurality of hydrogel beads surrounded by a membrane. The prosthesis is implanted in a dehydrated state and then hydrated to fill the intradiscal space.
Some prostheses include a constraining jacket to hold the prosthetic nucleus pulposus in place. However, such implants are large and require a large access point for insertion. To avoid this problem, implants have been proposed that rely on a composition that is inserted as a liquid and then hardens into a solid. U.S. Pat. No. 6,443,988 to Felt et al., for example, discloses an implant which includes a container that is inserted at the site of implantation and then filled with a material which is then cured in situ. The shape of this implant may be manipulated in situ and this implant may avoid problems of size and shape which would otherwise hinder implantation.
A similar prosthetic nucleus pulposus is disclosed in U.S. Pat. No. 6,187,048 to Milner et al. which discloses a spinal disc implant comprising a composition of acrylates which is injected into the intradiscal space and induced to at least partially polymerize through the addition of a cross-linking agent. This prosthesis, however, is similar in composition to joint implants, which eventually decompose and become mobile.
WO 01/68721 to BioCure, Inc. discloses a composition useful for tissue bulking that includes macromers having a backbone of a polymer having units with a 1,2-diol and/or 1,3-diol structure. Such polymers include poly(vinyl alcohol) (PVA) and hydrolyzed copolymers of vinyl acetate, for example, copolymers with vinyl chloride or N-vinylpyrrolidone. The backbone polymer contains pendant chains bearing crosslinkable groups and, optionally, other modifiers. The macromers form a hydrogel when crosslinked. This composition can be injected as a liquid and crosslinked into a solid hydrogel in situ.
The composition taught in WO 01/68721 results in a hydrogel that is suitable for many bio-applications. However, the hydrogel does not have the properties necessary for the particular and repeated stresses placed on a spinal disc nucleus pulposus.